Animated GIFs, journal entries, cat photos, and now, more ads. Tumblr, in an effort to further boost its revenue, is rolling out ads into users' mobile feeds.

The privately held social-networking and blogging site introduced sponsored posts to its desktop service less than a year ago. These ads have allowed fashion, entertainment and other brand partners to pay to have their posts appear in Tumblr's "Radar" right rail. Radar ads are responsible for more than 120 million daily impressions, Tumblr says.

"We're incredibly proud of our partners' creativity and have been constantly amazed by how well these creations can fit into our Dashboards," Lee Brown, Tumblr's vice president of sales, said Monday in a blog post. "So today we're bringing these posts over to our mobile apps," he said.

The mobile ads program currently comprises six partners including ABC Entertainment and ABC Family, GE, Pepsi and Warner Brothers. Android and iOS users will know what they're looking at is an ad by the animated "$" sign appearing in the top-right corner of the post.

Tumblr, a site designed for personal and artistic expression, is aiming to make the corporate ads integration seamless. Only "every now and then" will people see promoted posts as they scroll through their mobile feeds, the site said. Or specifically, up to four times a day, the company noted.

That characterization of the ads' frequency appears accurate -- a quick scroll-through of the Tumblr app on iOS revealed relatively few promoted posts.

But more ads are likely on the way, with Monday's announcement marking "the first of many" planned Tumblr ad launches in 2013 "to empower marketers to engage with and add to the most vibrant community online," Tumblr's Brown said in a statement.

Still, the hope seems to be that users will engage with the mobile ads in the same way they interact with user-generated content on Tumblr: by commenting, "liking" and re-posting to their own blogs.

The fashion, entertainment and brand partners that Tumblr already has on the desktop have generated "tens of millions" of notes or comments on their posts, Tumblr said.

However, "Tumblr users will likely be ambivalent about the addition of ads to their dashboard feeds," said Brad Flora, president and co-founder at Perfect Audience, a San Francisco-based ads placement company. "No one likes to have advertising pop up suddenly, but, the demo ads Tumblr is showcasing look amazing, and that makes all the difference," he said.

Sorting out the best ways to monetize its site will be important for Tumblr as competition between it and other social networks like Facebook and Twitter heats up in mobile markets. On mobile, Tumblr's own user base has quadrupled over the past six months, the company said. The site has 54 million users total.

Just last week, for instance, Twitter announced a new program to let marketers target their ads to specific tweeted keywords.

Over the first week of the mobile ads program, the premiere placements will belong to Warner Brothers, which will showcase teaser trailers for the upcoming films "The Great Gatsby" and "The Hangover: Part III," and GE, which will spotlight its jet engines and locomotives, Tumblr said.